The present invention relates to an epitaxial wafer of gallium arsenide phosphide that is prevented or inhibited from cracking due to internal stress. In the present disclosure, it is understood that the gallium arsenide phosphide is represented by GaAs.sub.l-x P.sub.x where x is a mixed crystal ratio greater than 1 but less than 0.5.
Epitaxial wafers of gallium arsenide phosphide--that are obtained by forming a mixed crystal layer of gallium arsenide phosphide on a gallium phosphide substrate by epitaxial growth technologies--have primary application in the fabrication of LEDs (light emitting diodes) that emit visible light lying in the red to yellowish green range.
A serious problem associated with such epitaxial wafers is that, because the substrates and epitaxial layers have varying crystal lattice constants, they are distorted due to a crystal lattice constant difference, so giving rise to some defects such as increased dislocation and cracking, and this is particularly true of when a desired mixed crystal layer of gallium arsenide phosphide is formed directly on a gallium phosphide single crystal substrate.
So far, this distortion has been relieved by forming on the gallium phosphide single crystal substrate a layer with the mixed crystal ratio, x, varying from 1 to a desired value, and then forming on that layer a layer having a desired constant mixed crystal ratio.
This method gives a prima facie satisfactory epitaxial wafer, but the resultant wafer is now found to be apt to bend. In order to prevent the bending of the wafer, the applicant has already proposed to define the "overshoot" of the mixed crystal ratio, as set forth in JP-A-3-201575.
Even with this technique for preventing the bending of the wafer by defining such "overshoot", it is unavoidable for stress to remain within the epitaxial wafer, and this is responsible for wafer cracking. The use of an easy-to-crack wafer thus offers a problem that the productivity of LEDs drops.
This is partly because there is a difference in the lattice constant between the substrate and the gallium arsenide phosphide mixed crystal, and particularly because of a difference in the coefficient of thermal expansion between them; that is, the epitaxial wafer is distorted, while it is cooled down to normal temperature upon being subjected to vapor phase epitaxial growth at a high temperature from 600.degree. C. to 1,000.degree. C., so that stress can remain within the epitaxial wafer.
In view of the problems mentioned above, a primary objective of the invention is to provide an epitaxial wafer of gallium arsenide phosphide which is less likely or unlikely to crack during the process of fabricating LEDs.